1. Field of the Invention
The present invention relates to drive systems for two pairs of wheels on a motor vehicle, with a differential associated with each pair of wheels and where at least one of the pairs of wheels can be steered. The steering angle and rotational speed of the steered pair of wheels can be determined by means of sensors, and a control circuit selectively operates differential locks associated with the differentials, depending upon the magnitude of the steering angle.
2. Description of the Related Art
West German Pat. No. 34 40 492 teaches an agricultural tractor having front and rear wheels each driven through a differential gear with a differential lock that can be controlled according to load. The drive to the front pair of wheels is taken through a selective clutch from a main transmission. A control circuit is provided for the control of the differential locks and the selective clutch, by means of which the timing and the magnitude of the friction locking of the differential locks is controlled. The control circuit compares reference values stored in its memory of the speed differences between the drive shafts for each set of wheels at all possible steering angles, and compares these to actual values in order to apply, remove or modulate the differential locks. However, the selective clutch can only be fully engaged or disengaged, and basically forms a rigid connection between the front and rear pair of wheels.
This drive system has the disadvantage that, although relative rotation of the wheels of one side with respect to the wheels of the other side during cornering may be accommodated by the control circuit, a stress remains between the front and rear wheels, since at any relative rotation above or below the optimum point the front wheels overrun or underrun the rear wheels. This stress could be relieved by opening the selective clutch by means of a control circuit, but then the flow of power to the front wheels would be interrupted and they would no longer contribute to propulsion of the vehicle.